The invention relates to wireless power transmission and particularly to the utilization of light sources in the transmission.
It is known to use wireless power transmission in situations where physical wiring or cabling between a power source and the power consumption point is difficult, if not impossible, to implement. Electromechanical devices conventionally employ inductive elements for power transmission between rotating or otherwise separated components. Inductive power transmission is also used for recharging low-power batteries, for example. In inductive power transmission- the power source and the power consumption or recharging point must be located close to each other and often it is necessary to isolate them galvanically from the environment, because the electromagnetic radiation caused by the induction may interfere with the operation of adjacent devices.
The prior art also knows solutions that employ radio frequency power transmission from a transmitter to one or more receivers, such as radio frequency identification and key cards. These solutions are only suitable for power transmissions of very low level, because it is difficult to direct the total power to be used at specific receiving antennas, and in practice transmission losses are extremely great. In addition, the efficiency of radio frequency transmission degrades rapidly as a function of distance.
Another solution to carry out wireless power transmission is to use a light source as the power transmitter, the transmitted light being then received by a photo-detector and converted into electric current. A light source is easier to direct towards a receiver and therefore a better efficiency can be gained than in radio frequency power transmission, for example. Publications EP 734 110 and U.S. Pat. No. 4,078,747, for example, describe solutions in which high power lasers are used for the wireless transmission of high currents in the power supply of electric trains and for transferring solar energy generated in space.
A problem with the above-described solutions is that they are not suitable for efficient wireless power transmission in environments occupied by people because the intensity of the laser is extremely dangerous. Even if significantly reduced, the power levels required for sufficiently good efficiency would be such that in case of eye exposure, the laser would at least severely damage vision.
It is therefore an object of the invention to provide an improved method and equipment implementing the method to allow the above problems to be solved. The objects of the invention are achieved by a method, system, transmitter and receiver which are characterized by what is stated in the independent claims.
The preferred embodiments of the invention are disclosed in the dependent claims.
An underlying idea of the invention is that power is transmitted wirelessly in a system comprising a power transmitter which in turn comprises a first light source, directing means for directing the light emitted by the first light source to a desired direction, and a second light source emitting light of a substantially lower intensity than that emitted by the first light source, the light emitted by the second light source being transmitted around the light emitted by the first light source and parallel to it. In addition, the system comprises one or more power receivers comprising a first photo-detector for receiving the emitted light and for converting it into electric current, a second photo-detector for detecting the light emitted by the second light source, and transmission means responsive to the detection to transmit a control signal to the power transmitter when the power receiver is receiving light emitted by the second light source. Consequently, when the power transmitter receives from the power receiver a control signal indicating that the power receiver is receiving low intensity light emitted by the second light source, the first light source of the power transmitter is switched on to transmit light surrounded by the light emitted by the second light source. If a disturbance is detected in the light emitted by the second light source, the transmission of the control signal from the power receiver stops and the first light source of the power transmitter is switched off.
An advantage of the method and system of the invention is that the low intensity light emitted by the second light source forms a xe2x80x98virtual insulatorxe2x80x99 around the higher intensity light emitted by the first light source whereby, if the virtual insulator is xe2x80x98brokenxe2x80x99, i.e. the light emitted by the second light source is blocked, the supply of the high intensity light is switched off immediately and therefore the light cannot cause damage. The procedure of the invention thus enables safe wireless power transmission by means of light sources. A further advantage of the invention is that the power receiver can be included in any device employing substantially low power, such as diverse office equipment, personal or entertainment electronics devices, a wireless power supply for which can be advantageously arranged from one power transmitter located in the same premises. A yet another advantage of the invention is that the power transmitter is arranged to automatically scan its surroundings to find receivers and to store the location of the receivers into its memory, whereby the directing of the transmitters at the receivers is rapid and power can be advantageously supplied alternately to multiple receivers. A still further advantage of the invention is that it enables power transmission of a significantly better efficiency than known solutions, i.e. a power transfer ratio of substantially at least 20%.